Digital carrier-recovery scheme for FM stereo detection

ABSTRACT

Systems and techniques for digital processing of FM signals. Digital processing may include recovering a carrier signal at a second frequency based on a reference signal at a first frequency.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is related to and claims priority to U.S. ProvisionalApplication Ser. No. 60/529,656, filed Dec. 15, 2003 and U.S.Provisional Application Ser. No. 60/531,302, filed Dec. 18, 2003, whichare hereby incorporated by reference.

TECHNICAL FIELD

This disclosure relates to digital techniques for FM stereo reception.

BACKGROUND

Public broadcast of radio is an important source of information andentertainment for people all over the world. The transmission of radioprograms is based on analog technology, typically using amplitudemodulation (AM), frequency modulation (FM), and stereophonic FM (alsoreferred to as FM stereo). In an analog FM system, an analog signal maybe encoded into a carrier wave by variation of its instantaneousfrequency in accordance with the input analog signal.

FM stereo was introduced to create a more natural listening experience.Rather than a single signal including all of the audio information,stereo transmission involves separate left (L) and right (R) signals.The received and processed L and R signals are sent to differentspeakers, reproducing (at least partially) the spatial location of thesource of a sound.

There are two systems for transmission of FM stereo defined by theInternational Telecommunications Union (ITU): the stereophonic multiplexsignal system and the pilot tone system. In the pilot tone system(according to the ITU standard), suppressed-carrier amplitude modulationis used to modulate stereophonic information onto a higher frequency,and that information can be combined with mono-compatible information inthe baseband to form a composite signal that is then frequency modulatedto the appropriate program channel. To detect the stereo signal, thecarrier that modulates the stereophonic information needs to berecovered.

SUMMARY

Systems and techniques described herein provide for digital processingof FM mono and stereo signals.

In general, in one aspect, a method of FM digital signal processing mayinclude receiving one or more digital signals including a first signalwith a first frequency. The method may include multiplying the firstsignal to obtain a second signal. Multiplying the first signal maycomprise squaring the first signal. The method may include filtering thesecond signal to obtain a high frequency component of the second signal,where the high frequency component may have a second frequency higherthan the first frequency. The second frequency may be twice the firstfrequency. In some implementations, the first frequency may be a 19 kHzfrequency for a pilot tone, and the second frequency may be a 38 kHzfrequency for a carrier signal.

Filtering the second signal may comprise filtering the signal with afilter of order n. Delaying the second signal may comprise delaying thesecond signal using a delay element having a transfer function ofZ^(−(n/2)).

The method may further include combining information indicative of thehigh frequency component of the second signal and the delayed signal toobtain a normalization factor. The method may include generating anoutput signal of the high frequency component of the second signal andthe normalization factor. Combining the high frequency component of thesecond signal and the delayed signal may comprise generating adifference signal.

The method may further comprise using the output signal to obtain astereophonic signal. The stereophonic signal may be a left and rightdifference signal. The method may further comprise obtaining separateleft and right signals using the left and right difference signal and aleft plus right signal.

In general, in another aspect, a computer program is operable to causeone or more machines to perform operations comprising multiplying dataindicative of a first signal having a first frequency to obtainmultiplied data. The operations may further comprise filtering themultiplied data, where the multiplied data may include data indicativeof a second signal having a second frequency greater than the firstfrequency. The operations may further include generating delayed data bydelaying the multiplied data.

In general, in another aspect, a carrier recovery system may comprise amultiplier having an input to receive one or more digital signalsincluding a first signal having a first frequency, the multiplierconfigured to generate a second signal by multiplying the first signal.The system may further comprise a high pass filter in communication withthe multiplier, the high pass filter to pass a high frequency componentof the second signal. The system may further comprise a delay incommunication with the multiplier, the delay configured to generate adelayed signal by delaying the second signal.

The system may further comprise a summer configured to sum the highfrequency component of the second signal and the delayed signal toobtain a normalization factor. The system may further comprise acombiner configured to generate an output signal using the highfrequency component of the second signal and the normalization factor.

In general, in another aspect, a carrier recovery system may comprisemultiplying means for multiplying one or more digital signals includinga first signal having a first frequency, the multiplying means therebygenerating a second signal. The system may further comprise high passfiltering means in communication with the multiplying means. The systemmay further comprise delay means in communication with the multiplyingmeans.

In general, in another aspect, a method of FM digital processing maycomprise receiving one or more digital signals including a first signalhaving a first frequency. The method may include obtaining a secondsignal by multiplying the first signal. The method may further includefiltering the second signal to obtain a high frequency component of thesecond signal. The method may further include generating a firstnormalization factor based on the second signal at a first time. Themethod may further include generating a second different normalizationfactor based on the second signal at a second time different than thefirst time.

The first time and the second time may be separated by a pre-selectedtime difference. The first time and the second time may be separated bya time difference determined based on one or more parameters of a radiosystem comprising a transmitter and a transceiver. The one or moreparameters may include a transmitter channel effect of the radio system.The one or more parameters may include a transceiver hardwarecharacteristic.

In general, in another aspect, a computer program may be operable tocause one or more machines to perform operations comprising multiplyingdata indicative of a first signal having a first frequency to obtainmultiplied data, the multiplied data including data indicative of asecond signal having a second frequency greater than the firstfrequency. The operations may further comprise filtering the multiplieddata to obtain the data indicative of the second signal. The operationsmay further comprise generating a first normalization factor based onthe data indicative of the second signal at a first time, and generatinga second different normalization factor based on the data indicative ofthe second signal at a second time different than the first time.

In general, in another aspect, a carrier recovery system may comprise amultiplier, the multiplier having an input to receive one or moredigital signals including a first signal having a first frequency,multiplier configured to generate a second signal by multiplying thefirst signal. The system may further comprise a high pass filter incommunication with the multiplier, the high pass filter configured topass a high frequency component of the second signal. The system mayfurther comprise a delay in communication with the multiplier, the delayconfigured to generate a delayed signal by delaying the second signal.The system may further comprise a summer configured to sum the highfrequency component of the second signal and the delayed signal toobtain a time-dependent normalization factor. The system may furthercomprise an output configured to generate a first output signal bycombining the high frequency component of the second signal and a valueof the time-dependent normalization factor at a first time. The outputmay be further configured to generate a second output signal bycombining the high frequency component of the second signal and adifferent value of the time-dependent normalization factor at a seconddifferent time.

In general, in another aspect, a carrier recovery system may comprisemultiplying means, the multiplying means having an input means forreceiving one or more digital signals including a first signal having afirst frequency, the multiplying means for generating a second signal bymultiplying the first signal. The system may further comprise high passfiltering means in communication with the multiplier, the high passfiltering means for passing a high frequency component of the secondsignal.

The system may further comprise delay means in communication with themultiplying means, the delay means for generating a delayed signal bydelaying the second signal. The system may further comprise summingmeans for summing the high frequency component of the second signal andthe delayed signal to obtain a time-dependent normalization factor. Thesystem may further comprise output means for generating a first outputsignal by combining the high frequency component of the second signaland a value of the time-dependent normalization factor at a first time,the output means further for generating a second output signal bycombining the high frequency component of the second signal and adifferent value of the time-dependent normalization factor at a seconddifferent time.

The details of one or more implementations are set forth in theaccompanying drawings and the description below. Other features andadvantages will be apparent from the description and drawings, and fromthe claims.

DESCRIPTION OF DRAWINGS

FIG. 1 is a conceptual FM stereo transmission spectrum.

FIG. 2 is a functional block diagram of a digital implementation of anFM stereo receiver.

FIG. 3 is a functional block diagram of an implementation of a digitalFM stereo baseband processor.

FIG. 4 shows a functional block diagram of an implementation of acarrier recovery module.

FIG. 5 shows an implementation of an FM stereo receiver system.

FIG. 6 shows an implementation of a control sequence that may be usedwith a receiver system such as that shown in FIG. 5.

Like reference symbols in the various drawings indicate like elements.

DETAILED DESCRIPTION

As noted above, a pilot tone FM stereo system uses frequency modulationfor a frequency division multiplexed baseband signal having astereophonic signal and a pilot tone. FIG. 1 shows a conceptual spectrumfor FM stereo transmission. According to the ITU specification, a pilottone system multiplexes the left and right audio signal channels tocreate a mono-compatible signal that is equal to the sum of the left andright channels (L+R). The mono-compatible signal is transmitted in thebaseband 110.

The difference of the left and right channels (referred to as L−Rherein; however the R−L may be used) is modulated usingsuppressed-carrier amplitude modulation with a carrier frequency 120 of38 kHz. A 19 kHz reference signal, which is referred to as a pilot tone115, is transmitted as well. Although not discussed herein, there areoptional auxiliary data transmission channels such as the SubsidiaryCommunications Authorization (SCA) channel that are generallytransmitted at lower power and higher frequencies (e.g., beyond 53 kHz).

Note also that although the currently used pilot tone and carrierfrequencies (19 kHz and 38 kHz, respectively) are discussed herein, thecurrent systems and techniques may be applied for frequencies differentthan those in current use.

Both the sum and difference signals may be pre-emphasized according tothe ITU specification. The L+R, L−R, and the pilot signals form amultiplexed signal that is then frequency modulated to the desiredcarrier frequency and transmitted. At the receiver, the 38 kHz carrierneeds to be recovered using 19 kHz pilot reference signal in order todetect the difference signal.

FIG. 2 shows a functional block diagram of a digital implementation ofan FM receiver 200. A radio frequency (RF) analog front-end 210 receivesan FM signal from an antenna 220 and transmits an analog signal to an FMchannel select filter 230, which filters out the desired programchannel. An analog-to-digital converter (ADC) 235 converts the resultinganalog signal to a digital signal. Note that the received analog signalmay be converted to a digital signal prior to selecting the desiredchannel, in some implementations.

The digital signal is demodulated using a digital baseband processor240, described in more detail below. One or more digital to analogconverters (DACs) 250 may then be used to transform the left and rightchannel bitstreams to the analog domain so that they may be played(e.g., the left and right analog signals may be used to drive speakers260).

A functional block diagram of an implementation of a digital FM stereobaseband processor such as processor 240 is shown in FIG. 3. An FMdemodulator 310 may receive the output bitstream of an ADC such as ADC235 of FIG. 2. Demodulator 310 extracts the multiplexed L+R, L−R, andthe reference pilot tone.

The 38 kHz carrier may be recovered using a carrier recovery module 320.Carrier recovery module 320 uses the pilot tone to recover the 38 kHzcarrier in order to detect the L−R bitstream. A detector 330 mayimplement (for example) bandpass and/or low pass filtering to detect theL−R bitstream.

A detector 340 may implement (for example) low pass filtering to extractthe L+R bitstream. Finally, the L+R and L−R bitstreams can be combinedappropriately using a combiner 350 to obtain the bitstreamscorresponding to the left and right channels. The output of combiner 350may be provided to one or more DACs, such as DAC 250 of FIG. 2.

FIG. 4 shows an implementation of a carrier recovery module such ascarrier recovery module 320 of FIG. 3. A bandpass filter 410 may be usedto obtain the 19 kHz pilot tone. A multiplier such as a squaring module420 may be applied to the filtered signal. The output of squaring module420 includes both a component at 38 kHz (twice the input signalfrequency) and a DC component, as Equation (1) illustrates:

$\begin{matrix}{{\cos^{2}\alpha} = {\frac{1}{2}\left( {1 + {\cos\; 2\alpha}} \right)}} & {{Equation}\mspace{14mu}(1)}\end{matrix}$

A high pass filter 430 may be used to filter out the carrier signal at38 kHz. Many possible implementations of high pass filters H(Z) may beused to recover the carrier signal.

Squaring (or other multiplication of) the input signal allows for therecovery of a signal at 38 kHz based on the 19 kHz pilot tone. However,magnitude of the L+R and L−R bitstreams may also need to be normalizedby determining a scaling factor for the squared input signal. Thebitstreams may need to be normalized because, e.g., the transmittergenerally scales the magnitude of the pilot tone to a lower power levelthan the transmitted audio signal.

Furthermore, transmission channel effects (such as a Doppler effectresulting from a moving transmitter and/or receiver) and the transceiverhardware implementation may cause the scaling factor (which may bedenoted as a(t)) to change with time. Squaring the pilot tone with ascaling factor can be represented as shown in Equation (2):

$\begin{matrix}{{{a^{2}(t)}{\cos^{2}\left( {2\pi\; f_{p}t} \right)}} = {\frac{a^{2}(t)}{2}\left( {1 + {\cos\; 2{\pi\left( {2f_{p}} \right)}t}} \right)}} & {{Equation}\mspace{14mu}(2)}\end{matrix}$

where f_(p)=19 kHz. To estimate the sampled scaling factor

$\frac{a^{2}(t)}{2},$denoted as c(k) in FIG. 4, a low-complexity low-pass filter can beimplemented using the combination of the high pass filter 430 (denotedas H(Z) in FIG. 4) and a delay element 440 (denoted as Z^(−m) in FIG. 4,where m=n/2 and n is the order of the filter H(Z)).

The output of filter 430 may be subtracted from the output of delayelement 440 using a summer 450. The output of summer 450 is c(k), whichmay then be inverted using an inverter 460. The output of filter 430 canthen be multiplied by 1/c(k) to obtain the recovered and normalized 38kHz carrier, using a multiplier 470.

As noted above, a(t) (or alternatively c(k)) may vary over time.Accordingly, in some implementations, the scaling factor may bedetermined a single time, while in others it is updated at least once,updated periodically, or updated generally continuously. For example, ifa(t) is changing slowly over time, the computation of c(k) may be doneoccasionally or periodically. However, if a(t) is changing appreciably,it may be advantageous to update a(t) continuously.

Other implementations of a carrier recovery module may be used. Forexample, depending on the overall FM stereo receiver architecturedesign, the correction factor may be passed onto the part of a basebandprocessor where the magnitude of the L+R and L−R bitstreams arenormalized. In an example of such an implementation, the L+R bitstreammay be multiplied by c(k) in order to avoid the division operationsrequired to compute 1/c(k).

Digital FM stereo signal processing may be performed using differentreceiver architecture implementations. FIG. 5 shows an implementation ofan FM stereo receiver system 500. System 500 may receive the output ofone or more ADCs such as converter 235 of FIG. 2. A filter 510 may beprovided in system 500 for additional channel selection and filtering,to reduce adjacent channel interference.

The output of filter 510 may be provided to a demodulator 520.Demodulator 520 may perform conventional digital FM demodulation. Forexample, demodulator 520 may obtain the demodulated multiplexed basebandsignal by computing the differential of the angle of the complexreceived signal from the ADC.

The output of demodulator 520 may be provided to an FM stereodemodulator system 530 for recovery of a mono signal (for monotransmission) or left and right signals (for stereo transmission). Insome cases, it may be advantageous to down-sample the signal receivedfrom the ADC. For example, depending on the particular FM demodulationalgorithm and sampling rate used, the signal may be down-sampled by afactor denoted k1 using a down-sampler 532, to reduce the complexityrequired for subsequent FM stereo demodulation.

In some implementations of system 530, the system may determine if thedemodulated signal includes a pilot tone. For example, the demodulatedsignal may be provided to a bandpass filter 534, and the output ofbandpass filter 534 at 19 kHz may be subsequently detected. If thedetected magnitude is greater than a threshold magnitude, the systemdetermines that the signal includes a pilot tone and thus detects FMstereo transmission. If the magnitude is less than the thresholdmagnitude, the system detects mono transmission. A stereo detectionindicator (SDI) may be set accordingly, to indicate stereo or monotransmission.

For stereo transmission, a carrier recovery module 536 may recover the38 kHz carrier so that the L−R bitstream can be down-converted tobaseband and subsequently detected. The output from carrier recoverymodule 536 and from down-sampler 532 (or alternately, FM demodulator520) may be multiplied using a multiplier 538.

The current inventor realized that a stereo receiver architecture withreduced complexity may be provided by using a common processing modulefor a mono signal and for both L+R and L−R signals. Alternatively, toincrease processing speed, more than one processing module may beprovided so that at least some of the signals may be processed inparallel.

For example, system 500 may include a processor 580 for processing mono,L−R, and L−R signals. A multiplexer 540 may receive the input frommultiplier 538 and from down-sampler 532. A channel select input 541determines whether the L−R bitstream or the L+R bitstream (or monobitstream, for mono transmission) is processed in processor 580.

For detecting both the mono and L+R transmissions, the FM demodulatedbitstream is first passed through a filter 542 which may implement bothlow pass filtering and notch filtering, where a notch at 19 kHz allowsthe mono or L+R signal to be extracted while rejecting interference fromthe pilot tone.

The filtered bitstream may be sub-sampled by a factor of k2 using asub-sampler 544. The bitstream may then be transmitted to a de-emphasismodule 546. De-emphasis module 546 may include a filter denoted by G(z),where G(z) can be derived as shown in Equation (3):

$\begin{matrix}{{G(z)} = \frac{1 - c}{1 - {cz}^{- 1}}} & {{Equation}\mspace{14mu}(3)}\end{matrix}$

where c=e^(1/τf), and where τ is typically equal to 50 μsec for Europeor 75 μsec for the United States.

The output of de-emphasis module 546 is input to a multiplexer 548. Formono transmission, a channel select input 549 (which may be based on thestereo detection indicator) sends the input signal of multiplexer 548 toboth L output 554 and R output 555 via output 551 of multiplexer 548.For stereo transmission, multiplexer 548 sends the input signal tooutput 551 to be combined with an L−R signal as described below.

For detection of the L−R signal, the output of multiplexer 540 is theinput from multiplier 538. The output of multiplexer 540 may beprocessed by processor 580 in the same manner as described above forprocessing the L+R or mono signals. The L−R signal is transmitted bymultiplexer 548 on output 550 to be combined with an L+R signal.

The L+R and L−R signals are combined as follows. To obtain the Rbitstream, the L−R signal is inverted and added to the L+R signal in asummer 552. To obtain the L bitstream, the L−R and L+R signals are addedusing a summer 553. The L and R bitstreams may then be output via leftoutput 554 and right output 555, converted to analog signals and used todrive separate speakers (not shown).

FIG. 6 is a flow chart illustrating an implementation of a controlsequence that may be used with a receiver system such as system 500 ofFIG. 5. An input signal may be filtered (605), for example, using a 19kHz bandpass filter. The output of the filter may be used to detect apilot tone (610). If a pilot tone is not detected, mono transmission isdetected (615). The mono signal is transmitted to both a left channeloutput (620) and a right channel output (625).

If a pilot tone is detected, carrier recovery may be performed (630).The recovered carrier may be used to detect the L−R bitstream (635). TheL+R bitstream may be detected (640). The L+R and L−R bitstreams may becombined to generate a L bitstream (645) that is transmitted to the leftchannel output (620), as well as to generate a R bitstream (650) that istransmitted to the right channel output (625).

A number of implementations have been described. Nevertheless, it willbe understood that various modifications may be made without departingfrom the spirit and scope of the invention. For example, somefunctionality described above and illustrated in the figures may beimplemented using hardware, using software, or using a combination ofhardware and software. Additionally, actions described in a certainorder may in some cases be performed in a different order. For example,analog to digital conversion and/or digital to analog conversion may beperformed at a different place in the signal processing than described.Accordingly, other implementations are within the scope of the followingclaims.

1. An FM digital processing method comprising: receiving one or moredigital signals comprising a first signal having a first frequency;obtaining a second signal by multiplying the first signal; filtering thesecond signal to obtain a high frequency component of the second signal;delaying the second signal to obtain a delayed signal; combininginformation indicative of the high frequency component of the secondsignal and the delayed signal to obtain a normalization factor; andgenerating an output signal of the high frequency component of thesecond signal and the normalization factor.
 2. The method of claim 1,wherein combining the high frequency component of the second signal andthe delayed signal comprises generating a difference signal.
 3. Themethod of claim 1, wherein multiplying the first signal comprisessquaring the first signal.
 4. The method of claim 1, wherein the firstsignal is a reference signal, and wherein the high frequency componentof the second signal has a second frequency equal to twice the firstfrequency.
 5. The method of claim 1, wherein the first frequency is 19kHz.
 6. The method of claim 1, wherein filtering the second signal toobtain a high frequency component of the second signal comprisesfiltering the second signal with a filter of order n.
 7. The method ofclaim 6, wherein delaying the second signal to obtain a delayed signalcomprises delaying the second signal using a delay element having atransfer function Z^(−(n/2)).
 8. The method of claim 1, furthercomprising using the output signal to obtain a stereophonic signal. 9.The method of claim 8, wherein the stereophonic signal is a left andright difference signal.
 10. The method of claim 9, further comprisingobtaining separate left and right signals using the left and rightdifference signal.
 11. A computer readable medium storing a computerprogram operable to cause one or more machines to perform operationscomprising: multiplying data indicative of a first signal having a firstfrequency to obtain multiplied data, the multiplied data comprising dataindicative of a second signal having a second frequency greater than thefirst frequency; filtering the multiplied data to obtain the dataindicative of the second signal; generating delayed data by delaying themultiplied data; combining the data indicative of the second signal andthe delayed data to obtain a normalization factor; and generating outputdata using the data indicative of the second signal and thenormalization factor.
 12. The computer readable medium of claim 11,wherein combining the data indicative of the second signal and thedelayed data comprises generating difference data.
 13. The computerreadable medium of claim 11, wherein multiplying data indicative of thefirst signal comprises squaring the data indicative of the first signal.14. The computer readable medium of claim 11, wherein the first signalis a reference signal, and wherein second frequency is equal to twicethe first frequency.
 15. The computer readable medium of claim 11,wherein the first frequency is 19 kHz.
 16. The computer readable mediumof claim 11, wherein filtering the multiplied data comprises filteringthe multiplied data by applying a filtering function corresponding to afilter of order n.
 17. The computer readable medium of claim 16, whereindelaying the multiplied data comprises applying a delay amountcorresponding to a delay element having a transfer function Z^(−(n/2)).18. The computer readable medium of claim 11, further comprisinggenerating stereophonic data using the output data.
 19. The computerreadable medium of claim 18, wherein the stereophonic data includes leftand right difference data.
 20. The computer readable medium of claim 19,further comprising obtaining separate left data and right data using theleft and right difference data.
 21. A carrier recovery systemcomprising: a multiplier, the multiplier having an input to receive oneor more digital signals comprising a first signal having a firstfrequency, the multiplier configured to generate a second signal bymultiplying the first signal; a high pass filter in communication withthe multiplier, the high pass filter to pass a high frequency componentof the second signal; a delay in communication with the multiplier, thedelay configured to generate a delayed signal by delaying the secondsignal; a summer configured to sum the high frequency component of thesecond signal and the delayed signal to obtain a normalization factor;and a combiner configured to generate an output signal using the highfrequency component of the second signal and the normalization factor.22. The system of claim 21, wherein the multiplier is implemented atleast partially in software.
 23. The system of claim 21, wherein themultiplier comprises a squaring module.
 24. A carrier recovery systemcomprising: multiplying means for multiplying one or more digitalsignals comprising a first signal having a first frequency, themultiplying means thereby generating a second signal; high passfiltering means in communication with the multiplying means, the highpass filtering means for passing a high frequency component of thesecond signal; delay means in communication with the multiplying means,the delay means for generating a delayed signal by delaying the secondsignal; a summing means for combining the high frequency component ofthe second signal and the delayed signal to obtain a normalizationfactor; and an output means for generating an output signal using thehigh frequency component of the second signal and the normalizationfactor.
 25. The system of claim 24, wherein the multiplying means isimplemented at least partially in software.
 26. The system of claim 24,wherein the multiplying means comprises a squaring means.
 27. A methodof FM digital processing, comprising: receiving one or more digitalsignals comprising a first signal having a first frequency; obtaining asecond signal by multiplying the first signal; filtering the secondsignal to obtain a high frequency component of the second signal;generating a first normalization factor based on the second signal at afirst time; and generating a second different normalization factor basedon the second signal at a second time different than the first time. 28.The method of claim 27, further comprising generating an output signalof the high frequency component of the second signal and the firstnormalization factor.
 29. The method of claim 27, further comprisinggenerating another output signal of the high frequency component of thesecond signal and the second normalization factor.
 30. The method ofclaim 27, wherein the first time and the second time are separated by apre-selected time difference.
 31. The method of claim 27, wherein thefirst time and the second time are separated by a time differencedetermined based on one or more parameters of a radio system comprisinga transmitter and a transceiver.
 32. The method of claim 31, wherein theone or more parameters of the radio system comprise a transmitterchannel effect of the radio system.
 33. The method of claim 31, whereinthe one or more parameters of the radio system comprise a transceiverhardware characteristic.
 34. A computer readable medium storing acomputer program operable to cause one or more machines to performoperations comprising: multiplying data indicative of a first signalhaving a first frequency to obtain multiplied data, the multiplied datacomprising data indicative of a second signal having a second frequencygreater than the first frequency; filtering the multiplied data toobtain the data indicative of the second signal; generating a firstnormalization factor based on the data indicative of the second signalat a first time; and generating a second different normalization factorbased on the data indicative of the second signal at a second timedifferent than the first time.
 35. The computer readable medium of claim34, the operations further comprising generating an output signal of thehigh frequency component of the second signal and the firstnormalization factor.
 36. The computer readable medium of claim 34, theoperations further comprising generating another output signal of thehigh frequency component of the second signal and the secondnormalization factor.
 37. The computer readable medium of claim 34,wherein the first time and the second time are separated by apre-selected time difference.
 38. The computer readable medium of claim34, wherein the first time and the second time are separated by a timedifference determined based on one or more parameters of a radio systemcomprising a transmitter and a transceiver.
 39. The computer readablemedium of claim 38, wherein the one or more parameters of the radiosystem comprise a transmitter channel effect of the radio system. 40.The computer readable medium of claim 38, wherein the one or moreparameters of the radio system comprise a transceiver hardwarecharacteristic.
 41. A carrier recovery system comprising: a multiplier,the multiplier having an input to receive one or more digital signalscomprising a first signal having a first frequency, the multiplierconfigured to generate a second signal by multiplying the first signal;a high pass filter in communication with the multiplier, the high passfilter configured to pass a high frequency component of the secondsignal; a delay in communication with the multiplier, the delayconfigured to generate a delayed signal by delaying the second signal; asummer configured to sum the high frequency component of the secondsignal and the delayed signal to obtain a time-dependent normalizationfactor; and an output configured to generate a first output signal bycombining the high frequency component of the second signal and a valueof the time-dependent normalization factor at a first time, the outputmodule further configured to generate a second output signal bycombining the high frequency component of the second signal and adifferent value of the time-dependent normalization factor at a seconddifferent time.
 42. The system of claim 41, wherein the multipliercomprises a squaring module.
 43. The system of claim 41, wherein thefirst time and the second time are separated by a pre-selected timedifference.
 44. The system of claim 41, wherein the first time and thesecond time are separated by a time difference determined based on oneor more parameters of a radio system comprising a transmitter and atransceiver.
 45. The system of claim 44, wherein the one or moreparameters of the radio system comprise a transmitter channel effect ofthe radio system.
 46. The system of claim 44, wherein the one or moreparameters of the radio system comprise a transceiver hardwarecharacteristic.
 47. A carrier recovery system comprising: multiplyingmeans, the multiplying means having an input means for receiving one ormore digital signals comprising a first signal having a first frequency,the multiplying means for generating a second signal by multiplying thefirst signal; high pass filtering means in communication with themultiplier, the high pass filtering means for passing a high frequencycomponent of the second signal; delay means in communication with themultiplying means, the delay means for generating a delayed signal bydelaying the second signal; summing means for summing the high frequencycomponent of the second signal and the delayed signal to obtain atime-dependent normalization factor; and output means for generating afirst output signal by combining the high frequency component of thesecond signal and a value of the time-dependent normalization factor ata first time, the output means further for generating a second outputsignal by combining the high frequency component of the second signaland a different value of the time-dependent normalization factor at asecond different time.
 48. The system of claim 47, wherein themultiplying means comprises squaring means.
 49. The system of claim 47,wherein the first time and the second time are separated by apre-selected time difference.
 50. The system of claim 47, wherein thefirst time and the second time are separated by a time differencedetermined based on one or more parameters of a radio system comprisinga transmitting means and a transceiving means.
 51. The system of claim50, wherein the one or more parameters of the radio system comprise atransmitter channel effect of the radio system.
 52. The system of claim50, wherein the one or more parameters of the radio system comprise atransceiver hardware characteristic.